Traditionally, to protect against lightning aircraft methods have included a low resistance pathway throughout the metallic bulk of the fuselage to dissipate the electrical energy. Metallized fiber reinforced structural materials have been used along the exterior surfaces of composite parts to provide a medium to rapidly dissipate the energy. Some of the present lightning protective structures, although feasible for use on spacecraft and some aircraft, are not feasible for use on high use commercial aircraft. This is due to the rigorous and continuously changing pressure, humidity, and temperature environment experienced by commercial aircraft, as well as the different cost and maintenance constraints associated therewith.
Testing has shown that under high use commercial aircraft operating conditions certain lightning protective structures tend to experience substrate microcracking and finish cracking making them more susceptible to corrosion and ultraviolet degradation. Microcracking is sometimes referred to as “weave telegraphing.” Weave telegraphing refers to when: (a) the visual irregularities in the finishes take on the appearance of the underlying weave pattern of the surface, (b) the pattern becomes more pronounced while in-service, and (c) there is formation and propagation of substrate and/or paint finish cracking. The stated microcracks tend to form due to repeated and extreme temperature, humidity, and pressure fluctuations. Microcracking occurs due to a number of factors including internal stresses from differences in coefficient of thermal expansion, as well as from non-optimum interface adhesion between components in composite systems.
The microcracks can extend into visual paint layers, which can result in appearance degradation and increased maintenance and inspection times and costs. increased maintenance such as paint repair, is needed not just for appearances, but also to identify when repainting is necessary to prevent ultraviolet degradation of the underlying organic materials. Increased inspection is needed not only to monitor corrosion, but also to ensure the microcracks have not adversely affected structural integrity. Thus, such structures are not always cost-effective for long-term use in the commercial environment.
One type of lightning protective structure includes a substrate layer, a metal mesh screen, and a non-structural outer film that may be reinforced with materials such as glass or polyester. The mesh can be a metal woven fabric, random mat, or perforated metal that is usually expanded. Depending on the metal and substrate an additional non-structural prepreg layer may be used for galvanic isolation to avoid corrosion between the base substrate and the metal mesh. Although this structure provides the desired lightning protection, it provides no structural benefit and contains multiple non-structural layers typically. Thus, the structure is inherently labor intensive and costly to produce. The weight of resin needed to encapsulate the mesh to prevent corrosion and provide a smooth surface can exceed the weight of the metal mesh and as a result is heavy. Also the mesh system can be susceptible to microcracking.
Another protective structure approach is to use a solid metal over composite material. This structure is also heavy and difficult to process without manufacturing defects, such as voids, when co-cured as a solid film or applied as a spray to the cured part. Spray processes such as aluminum flame spray have the added complication of requiring qualified personnel and equipment typically not available at airline facilities.
Thus, there exists a need for an improved lightning protective structure for an aircraft that does not exhibit the above-mentioned disadvantages and provides the corrosion resistance, rain erosion resistance, environmental durability, structural performance, and electromagnetic protection including lightning protection characteristics desired.